1. Field of the Invention
The present invention relates to an ink jet recording head and an ink jet recording apparatus having said head.
2. Related Background Art
There are various recording methods for use with printer or facsimile apparatuses, such as a thermal method, a wire dot method or an ink jet method.
Among them, an ink jet recording method (thereinafter also referred to as "liquid jet recording method") is an effective recording method because of its relatively high recording speed, relatively low noise in recording, and excellent coloring capability. With this method, various constitutions for the discharging device thereof have been proposed in recent years, and improved for the practical use.
This ink jet recording method is one in which the recording liquid, called the ink, is discharged as fine droplets based on one of various discharge methods to stick to a recording medium such as paper for the recording. There are various methods of discharging the ink, but among them, a method of discharging the ink by the use of the heat energy has been noted recently. That is, with the ink jet recording method, the liquid, to which the heat energy is applied, is caused to undergo a state change, causing the occurrence of bubbles, and is discharged through discharge ports of the recording head due to an action force based on the state change. The recording operation is performed by discharging droplets which then stick to the recording medium.
The ink jet recording method has a feature of providing a quality image with high resolution at a fast speed, because it is not only applied quite effectively to the so-called drop-on-demand recording method, but also allows a recording head with high density multi discharge ports to be easily implemented.
A typical exemplary recording head for the apparatus to which the ink jet recording method is applied comprises discharge ports provided for discharging the liquid, ink channels each communicating with a respective discharge port and having a heat acting portion where the heat energy for use in discharging the liquid can act on the liquid, and heat energy generators as means for generating the heat energy each of which is provided to correspond to the heat acting portion.
A typical example of the heat energy generator is an electricity-heat converter comprising a pair of electrodes, and a heat generating resistive layer connected to those electrodes and having the area (heat generating portion) where the heat is generated between those electrodes.
FIG. 1 is a typical top view showing a substrate 20 on which a heat energy generator is formed according to background art. In FIG. 1, on the substrate 20 is formed, in addition to a heat energy generator 21, temperature sensors 22 for detecting the temperature of the recording head, subheaters for heating the recording head based on the head temperature detected by the temperature sensors 22 to control the temperature, and wire bonding pads 26. Further, as the substrate 20 is in contact with the ink, for example, a tantalum (thereinafter denoted as "Ta") layer 24 is provided for the uppermost layer as the ink-resistant protective layer and the anticavitation protective layer. The Ta layer 24 covers an area other than the area where the wire bonding pads 26 for holding the electrical connections to the external are disposed, that is, all of the substrate on the discharge port side (on the heat generator side of electricity-heat converter) on the opposite side of the boundary 24B in FIG. 1. Note that the ink exists within the ink channel and the common liquid chamber as the recording head is constructed by the connection of the substrate 20 and the ceiling plate, so that the contact portion of the ink with the substrate 20 is an area on the discharge port side of the boundary 25B in FIG. 1. Also, in FIG. 1, the temperature sensors 22 are disposed outside of the boundary 25B of the liquid chamber.
A typical cross-sectional view of the substrate 20 taken along the line A-B in FIG. 1 is shown in FIG. 2. As shown in FIG. 2, the Ta layer 24 covers all the substrate except for the area where the wire bonding pads 26 are disposed, or almost all the surface of the substrate. Further beneath it, SiO.sub.2 layers for the insulation, A1 electrodes 27, and heat generating resistive layers 27a composed of HfB.sub.2 are formed, and within N-type Si, the functional elements of a P layer and an N layer are fabricated. However, in the related background art as above described, there were some cases in which the layer on the substrate 20 might be electrically broken. With this breakdown, it could be found that a specific breakdown mode frequently occurred. That mode is shown in FIG. 3.
This breakdown mode is one in which an insulating layer between A1 wiring electrodes 27 and the Ta layer 24 which is the uppermost layer undergoes a breakdown to cause a short-circuit, so that the Ta layer 24 itself becomes molten with the heat of the short-circuit. The breakdown area is shown in the area within circle D. This can be thought of in the following way. That is, electrical charges due to the static electricity stored in some portion of the recording head are passed via the wire bonding pads 26 into the A1 wiring 27 to be accumulated therein, so that the potential of the A1 wirings become quite high. On the other hand, as the Ta layer is an electrical conductor which tends to be a ground voltage when placed into contact with the ink, the Ta layer 24 is rarely charged with quantity of respect to the electrical charges therein. As a result, a quite strong electric field will be produced on the insulating layer SiO.sub.2 between the Ta layer 24 and the A1 wiring 27. If this electric field is stronger than a certain value, a discharge may arise between the Ta layer 24 and the A1 wiring 27, thereby causing the breakdown. The Ta layer 24 becomes molten with the heat generated at this time, so that the A1 wiring 27 is exposed. The breakdown as shown in FIG. 3 can be though to occur as above described.
The intensity of electric field formed between the Ta layer and the A1 wiring depends on the electrostatic capacity between the Ta layer and the A1 wiring. Accordingly, it is conceived that if the area of the Ta layer is reduced as above, the intensity of electric field formed may be smaller, so that the breakdown due to such a discharge is less likely to occur. Furthermore, a reduced area of the Ta layer as above described reduces the probability of the breakdown occurring on the substrate.
In addition, the breakdown mode has a positional dependence, which allows potential sites for the breakdown to be classified into more and less likely sites likely site of the breakdown. The more is the portion of around the temperature sensors 22 and subheaters 23, and the step portion of the A1 wirings connected to them, as shown in FIG. 1. Moreover, it seems that the breakdown is less likely to occur in a central portion of the substrate 20.
Although the details will be described later, the present invention has been developed based on such a view of the inventor.
FIG. 20 is a typical cross-sectional perspective view showing a liquid jet recording head. On a substrate 1102 are formed electricity-heat converters consisting of heat generating resistive layers having heat generators 1103, electrodes 1104 connected thereto, and liquid channel walls, via the semiconductor fabrication processes such as etching, vapor deposition or sputtering, on which a ceiling plate 1106 is further provided. The recording liquid (ink) 1112 is supplied from a liquid reserving chamber, not shown, through a liquid supply tube 1107 into a common liquid chamber 1108 of recording head 1101. In the figure, 1109 is a connector connected to the liquid supply tube. The liquid 1112 supplied into the common liquid chamber 1108 moves into liquid channels 1110 with the so-called capillary phenomenon, and is held stably by forming the meniscus at the discharge port communicating to the liquid channel. Here, by energizing the heat generators of the electricity-heat converters 1103, the liquid on the heat generators is rapidly heated, producing bubbles in the liquid channels, so that the liquid is discharged through discharge ports 1111 due to expansion and shrinkage of bubbles while forming liquid droplets. And a multi discharge port ink jet recording head is formed at a high discharge port array density of 16 nozzles/mm, i.e., with 128 or 256 discharge ports, or with a plurality of discharge ports arranged over an entire recording width of recording medium.
The electricity-heat converter is conventionally formed with various protective films, thereby causing the life of electricity-heat converter to be lengthened and the liquid to be stably discharged. The constitution of the protective layer is as described in Japanese Patent Laid-Open Application No. 59-194866, for example. In each figure of FIGS. 16 to 19, the head structure as disclosed in the same patent is shown. In each figure, on carriers 405, 505 are formed under layers 406, 506, and further heat generating resistive layers 407, 507, on which common electrodes 404, 504 and selective electrodes 403, 503 are provided. 402, 502 are heat generators of electricity-heat converters (thereinafter referred to as "heater"). On the heat generators formed in this way are laminated inorganic first protective layers 408, 508 made of SiO.sub.2, for example, on the upper face of which are laminated further inorganic second protective layers 409, 509 made of Ta, for example, and organic third protective layers 411, 511, if necessary, in which each protective layer is generally formed not only on the upper portion of electrode, but also on the entire face including the upper portion of under layer. Note that the liquid chamber is generally constituted of a first inorganic protective layer and a third organic protective layer (for example, "Photoneath" made by Toray Industries, Inc., "PIQ" made by Hitachi, Ltd.) laminated sequentially, and further a second inorganic protective layer may be laminated thereon, if necessary.
However, there are following drawbacks associated with this conventional example.
(1) During the fabrication process, when Ta or SiO.sub.2 is etched, pin holes may arise so that the electrical leakage between the wiring and the ink tends to occur, thereby decreasing the yield for the fabrication.
(2) Since the second inorganic protective layer is provided over the heating portion or the liquid chamber section in some cases, the adhering force of the film might be sometimes decreased because of increased film stress. As a result, the life of a liquid jet recording head is sometimes shortened because of larger thermal damage that occurs in the discharging operation.
By the way, with this ink jet recording head, the ink forms bubbles and is discharged by energizing the heater, which may be damaged by the cavitation when the bubbles produced in the ink disappear. Therefore, an anticavitation layer is provided over the surface of the heater on the liquid channel side, so as to prevent the heater from damage due to such cavitation. The second layer in FIG. 20 corresponds to this anticavitation layer.
Conventionally, the anticavitation layer was formed, with the sputtering method, under film formation conditions where the internal stress was almost zero, as the strength of the anticavitation layer would decrease if the stress remained within the inside of the anticavitation layer. However, with the conventional ink jet recording head as above mentioned, there is the problem that the durability of the anticavitation layer might vary greatly among heads having the anticavitation layer formed even under the same film formation conditions. For example, when the heater was repeatedly energized 10.sup.8 times, the survival rate of the anticavitation layer was between 50 to 100%, so that the reliability of the ink jet recording head might not be assured.